The invention relates to a device and a method for printing markings on flat items (also referred to as consignments) wherein the items are conveyed to a print head and during printing are exposed to a vacuum force.
Mechanical printers can be used to cancel stamps, wherein the stamp layout, however, is very limited. Alternatively, it is possible to use non-contacting printing systems, in particular ink jet printers, with which high printing qualities can be achieved and for which a free design of the stamp layout is possible. Of advantage with this system is the lower wear as compared to mechanical systems. Particularly attractive is the use of printing systems that permit a free design of the stamp layout if stamps are to be canceled on one side of a consignment and advertisement is to be printed onto the same side or on the other side. In such a case, the advertisement logos can change from consignment to consignment, so that different clients can be addressed, depending on the point of destination.
In order to achieve a high printing quality, it is necessary with ink jet printers that the consignments are transported past the print heads at a preset distance, e.g. 0.5 mm, and with as little disturbance as possible. It is important here that the consignments are transported at a predetermined speed that is synchronized with the control for the print heads. The problem of a speed difference between the conveying belts and the consignments exists, in particular if the consignments are transported via conveyor belts that are arranged in pairs one above the other, between which the consignments are pulled along frictionally engaged. Such a speed difference is particularly disadvantageous if a uniform marking must be applied to the consignment by different print heads, which are arranged one after another and which print with a time delay.